Ken, we use something similar but mounted on a larger machine.

It actually takes very little to push over a tree. If you have ever seen a 'tree feller' working ('Ax Men' on the TV for instance) you would see that a pair of plastic wedges about 2" thick at the base knocked into the cut with a small axe will easily flip a 24" tree over.

The amount of pressure that the bucket dump circuit can muster, lifting the front of a tractor for instance, is a lot more than you would first think.

The key of course is to remember that it is gravity after all doing the real work, if the tree is already leaning badly, this won't change the facts that it will likely fall that way despite your best efforts to the contrary.

If all you want to cut is the smaller stuff, say up to 12" I'd say even a fairly small machine would do it. Especially if they grew crowded and were straight as a pool cue.